Tips to resync audio recorded on audio cassettes?

Posted by yuangen 
Tips to resync audio recorded on audio cassettes?
February 18, 2009 11:11AM
Hi everyone,

we have a plan to dub some 60 video recorded in VHS tapes with audio recorded in audio cassettes, both are transfered to digital files, yet so far what we can do is matching MANUALLY the waveforms from audio cassette to that from VHS tape. Because VHS tape has different rpm from audio cassettes, we need to work on short intervals like every minute or 30 seconds, and find it not a very effective solution. We are not professionals of the field so any of your experience will be precious for us. The content are simply lectures of our master (male voice
solo from the beginning to the end), yet we do have like a whole year to work on this project. Thank you.
Re: Tips to resync audio recorded on audio cassettes?
February 18, 2009 11:42AM
Generally audio cassettes deck record speeds tended to vary with the AC (or batterie power) connected to them. Basically the speed varied over time.

You may get extremely lucky and find that speed change over time was a constant rate in which case you can just time shift the entire recording until things get close.

Unless you had a very high end professional audio cassette deck in which the speed was regulated internally to account for voltage fluctuations, what you're doing as about all you can do.

BTW you could even get drift over time with DAT decks.

Unless the clocks are locked and/or voltages are regulated, you'll get drift.

Since the drift fluctuates over time it may even be apparent on 30 second or 1 minute intervals if you were to use the both the VHS and cassette audio. Depending on the severity it can range from a phasing or flanging sound to "echo" to discrete repeats. So you'd have to delete the VHS audio to eliminate that issue.

In the years before the "digital" age I always recommended against using audio cassettes decks (unless of the voltage regulated or speed lock sync variety). One would be better off with a good mic feed to the video camera. The cost of the mic is almost always less than the cost of the time in post having to try to sync to audio cassette.
Re: Tips to resync audio recorded on audio cassettes?
February 18, 2009 04:15PM
Hi Craig, Thanks a lot for your prompt reply.

Indeed we will replace the audio recorded on VHS with that of audio cassette because the later was feeded directly from wireless mic, so the voice is much clearer.

So do you suggest we shall still work MANUALLY but at shorter intervals like 10 secs?
Re: Tips to resync audio recorded on audio cassettes?
February 18, 2009 04:49PM
You'd have to do this manually unfortunately.
Part of the issue is one can't predict the distance because the variation in speed of the cassette deck recording. In fact that distance can change over a long recording.

The best thing to do is have a "rule of thumb" so that once you see the waveforms vary by more than half a frame (for an example) or by .0167 (1/60th) of a second then it's time to pull up. It's best to pull up at pauses or ends of sentences to ensure they're not obvious to the ear.

In the future the ideal would be to have the wireless mic feed the mic input of the camera.
Re: Tips to resync audio recorded on audio cassettes?
February 21, 2009 03:12PM
I think I got the idea about what we shall do, yet I have some more basic questions on the pratical level. Saying that I have a file and an audio.wav file from audio cassette, and the result I need is the video combined with the audio:

a. Can I just dub it in Soundtrack Pro or shall I open a FCP project first with the video, then send it to Soundtrack Pro ?
b. It seems I can drag the audio file into track 1 then use "zoom in" and "vertical zoom in" in order to match audio waveforms, setting marks for editing and use some tool to stretch the wave forms, is such a correct way?
c. At last, after I finishing the job and delete the original audio, how can I save the video file now dubbed with the audio?
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